1. Field of the Invention
The present invention relates to at least partly transparent products made of plastic, especially products of high optical quality, equivalent to that of a glass window. It is conceivable to replace glass sheets with plastic sheets in the construction of windows, for example for buildings or transport vehicles.
2. Description of the Background
Compared with glass, plastics are lower in weight, which is a key advantage in the case of electrically driven town vehicles, in so far as an increase in their range is crucial. In such vehicles, it could even be envisaged producing, from plastic, in a single unit, entire doors or even complete sides of the body, including the windows, and, optionally, painting a lower part thereof. In general, the low weight of the transparent surfaces is relatively advantageous in the case of modern transport vehicles in so far as technical progress is going hand in hand with integration into windows of still more numerous functions (heated rear window, radio antenna, windscreen de-icing, colouration in order to prevent the passenger compartment heating up in strong sunlight, incorporation of electrochromic compounds, display of information on the windscreen, etc.) and with ever increasing amounts of glazed surfaces. This results in a general increase in the vehicle weight, to the detriment of its energy consumption.
Moreover, compared with glass, plastics are capable of providing improved safety conditions and superior anti-theft protection because of their greater toughness.
A not insignificant advantage of plastics compared with glass resides in their superior ability to be easily converted into complex shapes.
Finally, the ability of plastic sheets to be deformed substantially reversibly makes it possible to envisage ways of fitting them into considerably simplified body openings by a snap-fastening mechanism, from the inside just as from the outside of the vehicle.
According to a first approach, flat plastic sheets are formed by extrusion, a component is cut to the required dimensions and fastened to a thermoforming device, a contact thermoforming operation is carried out with at least one solid mould surface and, optionally, with the aid of compressed air or suction. The optical properties of a sheet thus extruded are not satisfactory.
Furthermore, the scratchability of plastics, mentioned previously, is such that, in their optical applications or as transparent elements, it is necessary to coat the shaped components with a hard varnish. This operation is accompanied, as is well known to those skilled in the art, by problems of the varnish flaking, these problems being more acute in the case of surfaces of complex shapes. In addition, it has only been envisaged forming the hard varnish at a temperature below the deformation temperature or softening point of the plastic, the shape of which is thus entirely preserved during this operation. Such conditions of forming the varnish are excessively restrictive and have resulted in considerable effort being expended to produce varnishes which form at sufficiently low temperatures and, at the same time, thermoplastics with high softening points. There therefore remained the need for a plastic which is transparent or intended for optical applications, in which the poor optical quality inherent in the extrusion technique and the problem of the varnish flaking would be avoided and in which many varnishes could be employed in combination with many plastics under satisfactory comparability conditions.
This product should be capable of being obtained by an inexpensive, reliable and simple process.